This invention relates to a carrier for an image developer, to an electrostatic latent image developer, to an image forming apparatus by electrophotography, electrostatic recording or electrostatic printing, to an image developing method and to a method of preparing a carrier.
In electrophotography, an electrostatic latent image formed on a photosensitive medium is developed by a developer. One-component developers composed of a toner and two-component developers composed of a toner and a carrier, such as glass beads and magnetic particles with or without resin coating, are known as the developer. The latter, two-component developers are suitably used for high speed printing and copying machines. In electrophotography of a digital-type in which a photoconductor is irradiated with a laser beam to form an electrostatic latent image, two-component developers are generally used for developing the latent image.
Recently, there is an increasing demand for a developer which can meet with requirements for high resolution, improved reproducibility in highlight and multi-color images. Thus, minimization of a minimum unit of latent images and high density thereof are desired. Accordingly, there is a great demand for a developing system which can accurately and precisely developing such a dot image. To attain this demand, various proposals have been made from the standpoint of both process conditions and developers (toners and carriers).
As to process conditions, minimization of developing gap, use of a thin film photoconductor and reduction of a beam diameter for writing are considered to be effective. However, these measures pose a problem of cost increase and reduction of reliability.
As for developers, the use of a small size toner will greatly improve the reproducibility of dot images but, in this case, occurrence of background stains and reduction of color density are caused. Additionally, when a small size toner is used for full color image formation in which a low softening point resin binder is used, adhesion of toner on the surface of the carrier occurs significantly. Thus, the developer is deteriorated during use to cause toner dispersion and background stains.
The use of a small size carrier will give the following merits.
(1) Because of a large surface area, every toner can be sufficiently charged by friction so that the formation of a low charging amount toner or a reversed charged toner can be minimized. Thus, background stains and toner dispersion or blurs of a dot image can be reduced so that the dot image reproducibility is improved.
(2) Because of a large surface area and reduced background stains, it is possible to reduce average charging amount of the toner. As a consequence, a high image density is obtainable. The use of a small size carrier can thus compensate demerits of a small size toner and is effective for obtaining desired properties of the small size toner.
(3) A small size carrier can form a dense but soft magnetic brush. Thus, a mark of brush is hardly formed in images.
In the case of known small size carriers, however, deposition or adhesion of carriers on a photoconductor is apt to occur during the developing stage so that injury of the photoconductor or an image fixing roller which is in contact with the photoconductor is apt to be caused.
In accordance with one aspect of the present invention, there is provided a carrier for an image developer for electrophotography, which comprises core particles, and a resin layer covering each of the core particles,
wherein the core particles have a weight average particle diameter of Dv which is 25-45 xcexcm and a number average particle diameter of Dp which meets with the following condition:
1xe2x89xa6Dv/Dpxe2x89xa61.3, 
wherein at least 70% by weight of the core particles have a particle diameter of less than 44 xcexcm, and
wherein the amount of the core particles having a particle diameter of less than 22 xcexcm is no more than 7% by weight.
In another aspect, the present invention provides a developer for electrography, which comprises a dry toner, and the above carrier.
In a further aspect, the present invention provides a method of developing an electrostatic latent image, which comprises contacting the latent image with the above develper.
The present invention further provides a developer container containing the above developer.
The present invention further provides an image forming apparatus having mounted thereon the above developer container.
The present invention further provides a method of preparing a carrier, which comprises the steps of:
sieving a carrier core material using an ultrasonically vibrated sieve to obtain core particles having a content of particles having a particle diameter of 22 xcexcm or less of 3% by weight or less; and
coating the core particles with a resin.
The present invention further provides a method of preparing a carrier, comprising the steps of:
coating core particles with a resin; and
sieving said coated core particles using an ultrasonically vibrated sieve to obtain carrier particles having a content of particles having a particle diameter of 22 xcexcm or less of 3% by weight or less.
It is an object of the present invention to provide a carrier for a two-component developer for electrophotography, which has excellent durability, which causes little carrier deposition and which can afford high quality images.
Another object of the present invention is to provide a two-component developer for electrophotography which can afford high density images free of background stains, which can give, with good highlight reproducibility, dot images having small variation of dot diameters and which can give images free of brush marks.